Primary structure of the active tryptic fragments of human and monkey salivary anionic proline-rich proteins.

The primary structures of the four human salivary anionic proline-rich proteins and an analogous protein from the saliva of a monkey (Macaca arctoides) have been further investigated. These proteins possess the unusual property of inhibiting crystal growth of calcium phosphate salts, and it has been proposed that they play an important role in the mouth, by preventing precipitation of calcium phosphate salts from the supersaturated saliva. The tryptic fragments responsible for this activity have been isolated from all five proteins and their complete amino acid sequences determined and compared. These active fragments have been unequivocally identified as the amino-terminal segment in all five proteins. The structures of the four human fragments are identical except for the presence of Asn at residue 4 in PRP-1 and -3 instead of Asp found in PRP-2 and -4. Comparison of the 30 residue human fragments with the monkey fragment shows 18 residues to be identical in these peptides, providing that residue 1 of the monkey fragment is aligned with residue 3 of the human proteins. Theses studies constitute the next step in determining the mechanism of action of these unusual proteins, and in determining their minimum chain length required for inhibitory activity.